in the beginning of this section one of the stickies has some really great reloading videos. you should check them out.
but to answer your question, you adjust and weigh one powder charge untill its the amount you want. then you weigh it a few more times just to be sure your adjustments on thepowder dispenser remain correct. you can now drop the powder rt into the case, or if you still want to you can weigh each one. really its up to you. the amount of powder to be used for any certin caliber, powder, and bullet combination are generally listed in reloading manuals and on some companies websites.
heres a fun site you can play on to kind of see what i mean. http://data.hodgdon.com/main_menu.asp
you can enter different calibers and play around to see all the info.